As a round or ellipse oxidation ditch, oxidation ditch comprises various ditches of double-channel one, triple-channel one, quadruple-channel one and so on, with a central island arranged in the center. Both sides and bottom of the central island are of concrete as well as the middle wall is of an integrated structure formed by segregating a ring girder of reinforcing steel bar and a brick wall. These ditches are generally arranged as an anaerobic area, an anoxic area and an aerobic area when designing oxidation ditch. Most of deposition tank equipped in the process is designed outside of the oxidation ditch, making the overall treatment device be incompact. For the purpose of saving investment and reducing area occupied, a treatment device came into being, wherein the deposition tank is designed at the central island position of oxidation ditch. As disclosed in the Chinese Patent No.CN03223656.5 that “A wastewater treatment device using A2/O oxidation ditch process”, the device consists of an anaerobic tank, an anoxic tank, an oxidation ditch and a deposition tank, wherein the combination of the deposition tank and the oxidation ditch form the tank body in the shape of concentric double-walled cylinder with the oxidation ditch around the outer side, the deposition tank inside and the anoxic tank located outside of the oxidation ditch as well as very near to the anaerobic tank outside of the anoxic tank. Although this design saves the space between tanks, and the connecting pipes in shared walls between tank bodies, the round deposition tank at the central island is an individual one. The objectives of deposition and separation will be realized only when sufficient volume of the deposition tank is ensured. Cost of the deposition tank accounts for 10-20% of the total costs of oxidation ditch. In terms of whole facility, defects of large area occupied and excessively-high costs still exist. In addition, the flexibility of use is inadequate and the whole device will be down in case of repairing deposition tank.
If the central island position is designed as a separate anaerobic area or a separate oxidation ditch, neither of these two forms is, from the design angle, rather reasonable no matter for area occupied or layout arrangement.